Magnetometer



Patented Aug. 30, 1949 UNITED STATES PATENT OFFICE 2,480,265 macna'roma'ran Jacob H. Rubensteln, Buffalo, N. Y.

Application January 23, 1946, Serial No. 642,835

8 Claims. 175-183) ,1 a This invention relates to a magnetometer for determining the direction or strength of a ma netic field, such as the magnetic field of theearth,

and which can therefore be used to determine the a magnetic meridian of the earth's field, and the angle of inclination or declination between this meridian and the axis of the apparatus.

This application is a continuation in part of my copending application Serial No. 562,728, filed November 9; 1944, now abandoned, for method and. apparatus for determining the direction or the core can be accomplished by the proper seleceter which is stable in its response.

intensity'of the lines of force of a magnetic field. I

One of the principal objects-of the invention is to provide such a magnetometer having an extremely high sensitivity.

Another object is to provide such a magnetometerwhich has alarge power output varying accurately with changes in the direction or intensity of a surrounding field, thereby to permit the use of the magnetometer to control itself or other instrumentalities such as an airplanein flight or indicating or recording instruments thereon.

Another purpose is to provide such a magnetometer which is simple and rugged in. construction and which will stand up under onditions of severe and constant use withoutgettlng out of order or adjustment.

Another'object-of 'the invention is to provide such a magnetometer which includes an oscillator whose output varies accurately and. sharply with changesin the strength or direction of the magnetic field surrounding the magnetometer.

Another object is to supply,'as a part of such.

an oscillator, as compared-with the usual'inductor used invacuum tube oscillators, a special inductor whose core can be readily biased to ,a flux density such that a small change in this density, caused by a change in the strength or direction of a surrounding magneticfield, will produce a large change in the permeability of the core, thus sharply changing the degree of feedback coupling or mutual inductance between the two sections of the inductor core and hence the amplitude of the oscillator output.

Another object is to provide in combination with such a special core an elongated antenna in the form of collecting plates or bars extending outwardly from the core of the inductor and attracting and collecting magnetic flux from a large area of the surrounding field to pass through the core thereby to provide a greatly increased flux to be measured.

. Another object-is to provide in combination with the core and antenna of such a magnetometer, permanent magnets so that the biasing of Other objects and advantages will appear from the following description and drawings in which: Fig. 1 is a side view of the head of a magnetometer embodying the present invention, this head being rotated in the magnetic field to determine the direction or intensity thereof.

portant step in accomplishing this is tomagnify as far as possible the flux change in the head as Fig. 2 is another side view of this head, this view being taken at right angles to Fig. 1.

Fig. 3 is a diagrammatic view showing the indu'ctor forming. part of the head shown in Figs. 1

and 2 combined in the grid circuit of an oscillator.

Fig. 4 is a, graph showing the output of the oscillator shown in Fig. 3 plotted against the flux passing through the core of the inductor and showing the sharp response which is obtained when the magnetometer is biased toward the saturation point of the core of the inductor.

Fig. 5 is a graph-showing how the permeability of the materials of which the. head is made varies as the flux density is raised from Zero toward saturation.

The invention is essentially directed to a magnetometer in which the magnetomotive force due to a magnetic field, such as. that of the earth, has a large-effect upon the'fiux passing through the magnetometer head b adding to or subtracting from the ma'gnetomotive force of the permanent magnets associated therewith depending upon the position of the head with respect to the field to be measured or upon the intensity of the field. In the magnetometer shown this flux is directed through a constricted core where it is measured-by first using its changes to alter the mutual inductance or coupling between two sections of a coil wound upon the core and then using these changes in coupling to change the magnitude of the output of an oscillator of which this coil is a part.

The invention is essentially concerned with a highly sensitive magnetometer in which minute changes in intensity or direction of the field to be measured will produce relatively large changes in'the electromotive force developed. A first imthe surrounding magnetic field ischanged in strength or direction and to direct substantially antennas i i,

3 all of this flux through a core of small cross section which will thereby contain such a high density of flux that its magnetic permeability will be sharply changed as the flux density is slightly changed.

To this end the magnetometer ofthe present invention includes the head shown in Figs. .1 and 2 which can be mounted to rotate in the magnetic field under measurement and which includes a core 10 fixed between two laminated path for the flux passing through the antenna array. Permanent magnets I2, I 3 are arranged at the outer ends of the antennas H in series aiding relation to each other so as to rotate with the head. The antennas H, H and core Ill are made of one of the special magnetic alloys, such as Permalloy which have high permeability, low coercive force and low hysteresis losses. In the graph, Fig. 5, is shown how the permeability of such alloys varies with flux density. It will be observed that at point A on the curve the permeability gets rapidly less as the flux density decreases whereas at point B it increases very sharply as flux density decreases. Since substantially the same flux passes through the antennas II that passes through the core II], it is obviously possible to produce a fiux density corresponding to A in the antennas and B in the core by merely selecting the proper relative cross sectional areas of the antennas and core,

this being done in the head illustrated in Figs.

1 and 2. It will be noted that the overall effective permeability of the entire head will be largely controlled by the permeability of the antennas ll rather than that of the core I because the length of the former is so much greater.

In the head forming the subject of the present invention the antennas II and core ID are so proportioned that when the head is subjected to the maximum magnetomotive force of the surrounding field to be measured, aiding that of the permanent magnets l2, l3, the overall permeability of the head will be approximately at A in Fig. 5. It will then be seen that any decrease in the strength of the surrounding field will cause a relatively large decrease in the total flux in the head, including the core l0, because the overall efiective permeability will go down as the net magnetomotive force goes down and both effects will act to reduce flux and act on each other in a cumulative degenerative manner. Conversely any increase in the net magnetomotive force on the head, when-the density is less than at A, will produce a very large increase in flux.

These relatively large flux changes will produce changesin the fiux density of the core Ill and since this core has been biased to point B on the curve, Fig. 5, a small reduction in its flux density produces a relatively large change in its permeability. With the head shown, therefore, the first important step of the invention, that of producing sharp changes in the permeability of the core ill by small changes in the net magnetomotive force applied to the head, has been accomplished.

Since the magnetomotive force of the permall so as to form a constricted nent magnets l2, I3 is constant the changes in permeability of the core ll becomes: function of the angle which the head makes with the surrounding field.

Therefore to utilize the head as an indicator of the strength or direction of a magnetic field it only becomes necessary to measure these changes of permeability.

To accomplish this it will be noted upon reference to Figs. 1 and 2 that a coil [5 is wound on the core in. This coil is divided in two sections I51: and I5!) to provide the inductor L of a vacuum tube oscillator as illustrated in Fig. 3. Many of the standard oscillator circuits could be used in carrying out the invention. However the oscillator shown, being of the type known as an electron coupled oscillator, has the advantages of large output with excellent stability.

In this form of oscillator, as in many others, the magnitude of the alternating electromotive force measured at the output is a function of the coupling or mutual inductance between sections I5a and [5b of the coil l5. Since both sections of this coil are wound on the core III, the degree of coupling or mutual inductance between them is a function of the permeability of the core. Hence the output of the oscillator depends on the permeability of the core l0 which in turn depends upon the strength or direction of the surrounding magnetic field.

In the electron coupled oscillator shown in Fig. 3, one side of the LC circuit is grounded and the other side connected directly to the grid I8 of the thermionic vacuum tube I9. The tube I9 is preferably in the form of a pentode having its suppressor grid grounded and having its plate 2| connected through the plate resistor or load 22 to the positive side of the B battery. The screen grid 23 forms the anode of the tube, this screen grid being shown as connected to the grounded side of the LC circuit by a condenser 24 and also to a tap of the B battery so as to maintain the screen grid or anode 23 at the correct D. C. operating potential. The connection to the cathode 25 is made through the tap 26 between the sections 15a and l5b of the inductor L to provide feedback or coupling. It will be observed that there are two adjustments provided-a variable resistor 28 in the cathode circuit and a variable condenser C' in parallel with the main condenser C of the LC circuit. Either or both of these are used to adjust the oscillator output as measured by the voltmeter 30 before putting the magnetometer into service.

With the inductor L of the head shown in Fig. 1 in the grid circuit of the oscillator as shown in Fig. 3 and the magnetic bias properly adjusted, I have found that a pronounced change occurs in the electromotive force output of the oscillator as the head is rotated in the earth's field or the strength of this field is varied. In Fig. 4 the volts measured at 30 are plotted against flux in oersteds passing through the core ID of inductor L. It will be noted that as the flux in the core is increased from zero no substantial change takes place until the flux density reaches a point where further increases make appreciable changes in the permeability of the core and hence in the degree of coupling between the two sections i511. and l'5b of the oscillator coil l5. Point B on Fig. 5 would be such a point.

In order'to bias the magnetometer head to such a point so that the oscillator output will vary between points M and N, Fig. 4, as the head is treme outward positions, the permanent magnets are moved toward each other until the output of the oscillator is just short of zero and are fixed at this point. Any reduction in the fiux in the core it caused, for example, by rotation of the head away from alignment with the field will then sharply increase the oscillator output.

It will be seen that, in the practice of the present invention, amplification of the effects of changes in the strength of the magnetic field to be measured or in the position of the magnetometer head with reference thereto has been obtained at several points: first, by magnetically biasing the core I so that the core contains a flux density such that a small reduction in this density will produce a relatively large increase in the core permeability; second, by proportioning the antenna members so that their flux density will produce an .overall head permeability vwhich will be decreased as this density is decreased; and third, by using a type of oscillator current induced by changes in the permeability which produces a large change in output elec-' lator or any one of several standard similar types,

the response of the magnetometer can be made to control a relatively large amount of power.

It should be noted that changing the permeability of the inductor core, besides changing the electromotive force of the oscillator output also changes the impedance across the LC circuit and the frequency of oscillation. Any or all of these effects could be used to measure the permeability change but I have found that measuring the changes in output electromotive force ofiers the simplest and most positive method.

With the extreme sensitivity resulting from all methods of amplification used and the ruggedness and power handling capabilities of the final response, the magnetometer formingthe subject of the present invention is capable of a wide range of uses to which less sensitive and less powerful magnetometers heretofore developed are not suited.

I claim as my invention: j

1. In a magnetometer, an inductor having a fixed core of high magnetic permeability, an elongated member of high magnetic permeability held in fixed relation at one end of said core for directing the varying flux from an external magnetic field into said core, said elongated member having a cross sectional area and length several times that of said fixed core and said proportions of said core and memberv being such that when said core is approaching saturation with flux the permeability of said member is approaching its maximum permeability, a condenser in circuit with the winding of saidinductor to provide an LC circuit, means resonating said LC circuit to maintain a, recirculating current therein, means maintaining a high flux density in said core short of saturation when the maximum amount of said fiux from said external field is directed into said core whereby a reduction in the amount of said flux from said external magnetic field so directed into said core causes a large increase in the permeability of said core, and means measuring changes in said recirculating of said core.

2.- In a magnetometer, an inductor having a fixed core of high magnetic permeability, an elongated member ofhigh magnetic permeability held in fixed relation at one end of said core for directing thevarying fiux from an external magnetic field into said core, said elongated memberv having a cross sectional area and length several times that of said fixed core and said proportions of said core and member being such that when imum amount of said flux from said external field is directed into saidcore whereby a reduction in the amount of said fiux from said external field so directed into said core causes a large in-' crease in the permeability of said core, and means measuring changes in the current characteristics of said LC circuit induced by changes in the permeabilityof said core.

3. In a magnetometer, an inductor having-a fixed core of high magnetic permeability, an elongated member of high magnetic permeability held in fixed relation at one end of said core for directing the varying fiux from an external magnetic field into said core, said elongated member having a cross sectional area and length several times that of said fixed core and said proportions of said core and member being such that when said core is approaching saturation with flux the permeability of said member is approaching its maximum permeability, a condenser in circuit with the winding of said inductor to provide an LC circuit, means resonating said LC circuit to maintain a recirculating current therein, means maintaining a high flux density in said core short of saturation when the maximum amount of said flux from said external field is directed into said core whereby a, reduction in the amountof said flux from said external magnetic field so directed into said core causes a large increase in the per-' meability of said core, and means measuring changes in the voltage drop of said recirculating current across said LC circuit induced by changes in the permeability of said core.

4. In a. magnetometer, a fixed core of high magnetic permeability and having a pair of windings wound thereon, an elongated member of high magnetic permeability held in fixed relation at one end of said core for directing the varying fiux from an external magnetic field into said core, said elongated member having a cross sectional area and length several times that of said fixed core and said proportions of said core and member being such that when said core'is approaching saturation with fiux the permeability of said member is approaching its maximum permeability, a condenser in circuit with one of said windings to provide an LC circuit, means supplying alternating current to the other of said windings at a frequency corresponding to the resonant frequency of said LC circuit to maintain a recirculating current in said LC circuit, means maintaining a high fiux density in said core short of saturation when the maximum amount of said flux from said external magnetic field is directed into said core whereby a reduction in the amount of said fiux from said external magnetic-field so directed into said core causes a large increase in uring changes in said recirculating current induced by changes in the permeability of said core.

5. In a magnetometer, a fixed core oi. high magnetic permeability and having a pair of windings wound thereon, an elongated member of high magnetic permeability held in fixed relation at one end of said core for directing the varying flux from an external magnetic field into said core, said elongated member having a cross sectional area. and length several times that of said fixed core and said proportions of said core and member being such that when said core is approaching saturation with fiux the permeability of said member is approaching its maximum permeability, a condenser in circuit with one of said windings to provide an LC circuit, means supplying alternating current to the other of said windings at a frequenc corresponding to the resonant frequency of said LO circuit to maintain a recirculating current in said LC circuit, means maintaining a high flux density in said core short of saturation when the maximum amount 01' said flux from said external magnetic field is directed into said core whereby a reduction in the amount of said flux from said external magnetic field so directed into said core causes a large increase in the permeability of said core, and means measuring changes in the voltage drop of said recirculating current across said LO circuit induced by changes in the permeability of said core.

6. In a magnetometer, an inductor having a fixed core of high magnetic permeability, an elongated member of high magnetic permeability held in fixed relation at one end of said core for directing the changing flux from an external magnetic field'into said core, means maintaining an alternating current in the winding of said inductor, said elongated member having a cross sectional area and length several times that of said fixed core and said proportions of said core and member being such that when said core is approaching saturation with flux the permeability of said member is approaching its maximum permeability, means maintaining a high flux density in said core short of saturation when the maximum amount of said flux from said external field is directed into said core, whereby a reduction in the amount of said flux from said external mag netic field so directed into said core causes a large increase in the permeability of said core,

and means for measuring the voltage drop of said alternating current across the winding of said inductor induced by variations in the permeability of said core, comprising a thermionic vacuum tube having a cathode and a grid connected by the winding of said inductor, and means measuring changes in the output current of said vacuum tube caused by said changes in said voltage drop.

7. In a magnetometer, a fixed core of high magnetic permeability and having a pair of windings wound thereon, an elongated member of high magnetic permeability held in fixed relation at one end of said core for directing the varying flux from an external magnetic field into said core, said elongated member having across sectional area and length several timesthat of said fixed core and said proportions of said core and member being such that when said core is approaching saturation with flux the permeability of said member is approaching maximum permeability, a condenser in circuit with one of said windings to provide an LC circuit, an oscillator including a thermionic vacuum tube having a grid circuit and an anode circuit means connecting said LC circuit in the grid circuit of said-thermionic vacuum tube,means connecting the other of said windings in the anode circuit of said vacuum tube to provide feedback to said LO circuit, and means measuring changes in the output current of said vacuum tube oscillator.

8. In a magnetometer, a fixed core of high magnetic permeability and having a pair of windings wound thereon, an elongated member of high magnetic permeability held in fixed relation at one end of said core for directing the varying flux from an external magnetic field into said core, said elongated member having a cross sectional area and length several times that of said fixed core and said proportions of said core and member being such that when said core is approaching saturation with flux the permeability of said member is approaching maximum permeability, a condenser in circuit with one of said windings to'provide an LC circuit, an oscillator including a thermionic vacuum tube having a grid circuit and an anode circuit means connecting said LC circuit in the grid circuit of said thermionic vacuum tube, means connecting the other of said windings in the anode circuit of said vacuum tube to provide feedback to said LC circuit, means measuring changes in the output current of said vacuum tube oscillator, and a permanent magnet at the extremity of said member remote from said core and maintaining a high flux density in said core short of saturation when the maximum amount of said flux from said external magnetic field is directed into said core whereby a reduction in the amount of said flux from said external magnetic field so directed into said core causes a large increase in the permeability of said core.

JACOB H. RUBENSTEIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PAIEN'I'S 

